MANILA: Some of the 1.4 million litres of industrial fuel oil inside a sunken Philippine tanker has started to leak into Manila Bay, the coast guard said on Saturday, as they raced to avoid an environmental catastrophe.

The MT Terra Nova sank in bad weather off Manila early on Thursday, killing one crew member and leaving the country potentially facing its worst oil spill disaster.

The oil slick has more than tripled in size and is now estimated to stretch 12-14km across the bay, which thousands of fishermen and tourism operators rely on for their livelihoods.

Divers inspected the hull of the vessel on Saturday and saw a "minimal leak" from the valves, coast guard spokesman Rear Admiral Armando Balilo said, adding it was "not alarming yet".

"It's just a small volume flowing out," Balilo said, adding "the tanks are intact".

"We're hoping that tomorrow we will be able to start syphoning the oil from the motor tanker," he said.

The ship that will carry the recovered oil is on its way to the area, he said.

The coast guard has warned that if the entire cargo leaked it would be an "environmental catastrophe".

It has previously said the oil leaking from the tanker appeared to be the diesel fuel used to power the vessel, which is resting on the sea floor under 34m of water.

The coast guard now thinks the slick is a mixture of diesel and industrial fuel oil.

Oil containment booms have been deployed for what Balilo earlier described as "the worst case scenario" of the cargo leaking out.

Three coast guard vessels were also spreading dispersants on the oil.

Balilo called for a suspension of fishing in Manila Bay to prevent people "eating contaminated fish".

The vessel sank nearly 7km from its origin in the port of Limay west of Manila. It was attempting to return to port after running into bad weather.

Sixteen of the 17 crew members were rescued from the tanker, which vessel tracking website vesselfinder.com said was 65m long and built in 2002.

The incident occurred as heavy rains fuelled by Typhoon Gaemi and the seasonal monsoon lashed Manila and surrounding regions in recent days.

The state weather service said the monsoon had weakened by late Friday, giving the authorities a window of relative calm at sea to recover the cargo.

The coast guard estimates the extraction would take at least seven days.

It met with representatives of the MT Terra Nova's owner and a contracted salvage company on Friday to discuss the timeline.

The Philippines has struggled to contain serious oil spills in the past.

It took months to clean up after a tanker carrying 800,000 litres of industrial fuel oil sank off the central island of Mindoro last year, contaminating waters and beaches of the island and devastating the fishing and tourism industries.

Another tanker sank off the central island of Guimaras in 2006, spilling tens of thousands of gallons of oil that destroyed a marine reserve, ruined local fishing grounds and covered stretches of coastline in black sludge.

 - AFP